undefined and null

Intro	Objects	The prototype
hasOwnProperty	Functions	How this works
Closures and references	The arguments object	Scopes and namespaces
Constructors	Equality and comparisons	Arrays
The Array constructor	The for in loop	The typeof operator
The instanceof operator	Type casting	undefined and null
Reasons against eval	setTimeout and setInterval	Automatic semicolon insertion

undefined and null

JavaScript has two distinct values for nothing, the more useful of these two being undefined.

The value undefined

undefined is a type with exactly one value: undefined.

The language also defines a global variable that has the value of undefined, this variable is also called undefined. But this variable is not a constant, nor is it a keyword of the language. This means that its value can be easily overwritten.

ES5 Note: undefined in ECMAScript 5 is no longer writable in strict mode, but its name can still be shadowed by for example a function with the nameundefined.

Some examples for when the value undefined is returned:

Accessing the (unmodified) global variable undefined.
Implicit returns of functions due to missing return statements.
return statements which do not explicitly return anything.
Lookups of non-existent properties.
Function parameters which do not had any explicit value passed.
Anything that has been set to the value of undefined.

Handling changes to the value of undefined

Since the global variable undefined only holds a copy of the actual value of undefined, assigning a new value to it does not change the value of the typeundefined.

Still, in order to compare something against the value of undefined it is necessary to retrieve the value of undefined first.

In order to protect code against a possible overwritten undefined variable, a common technique used is to add an additional parameter to an anonymous wrapper, that gets no argument passed to it.

var undefined = 123;
(function(something, foo, undefined) {
// undefined in the local scope does
// now again refer to the value
})('Hello World', 42);

Another way to achieve the same effect would be to use a declaration inside the wrapper.

var undefined = 123;
(function(something, foo) {
var undefined;
...
})('Hello World', 42);

The only difference being here, that this version results in 4 more bytes being used in case it is minified and there is no other var statement inside the anonymous wrapper.

Uses of null

While undefined in the context of the JavaScript language is mostly used in the sense of a traditional null, the actual null (both a literal and a type) is more or less just another data type.

It is used in some JavaScript internals (like declaring the end of the prototype chain by setting Foo.prototype = null), but in almost all cases it can be replaced by undefined.

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